BPL has lately attracted considerable attention as a starting material of heat resistant engineering plastics. Known processes for preparing BPL include (1) alkali fusion of diphenyl disulfonic acid, (2) hydrolysis of dibromodiphenyl, and (3) oxidative dimerization of 2,6-di-t-butylphenol followed by debutylation. The former two processes not only need severe reaction conditions but have difficulty in separating the product from a large quantity of inorganic salts used in the reaction. Accordingly, the process comprising oxidative dimerization of 2,6-di-t-butylphenol is the most preferred for industrial production of BPL.
This process, however, involves problems discussed below which are accompanied by debutylation subsequent to the oxidative dimerization. It is known that 3,3',5,5'-tetra-t-butyl-4,4'-dihydroxybiphenyl (hereinafter abbreviated as TBBPL) which is obtained by oxidative dimerization of 2,6-di-t-butylphenol can be debutylated by using an acid catalyst, e.g., sulfuric acid and p-toluenesulfonic acid, in an organic solvent. Consecutively proceeding, the debutylation reaction does not complete when conducted at a low temperature, and intermediate products, such as mono-, di-, tri-, or tetra-t-butyldihydroxybiphenyl remain in the reaction mixture, resulting in a reduction of BPL yield. Therefore, the debutylation reaction is usually carried out in a high-boiling solvent at a temperature elevated up to 200.degree. C. or more in order to complete the reaction. According to the process disclosed in JP-A-61-200935 or JP-B-62-135 (the terms "JP-A" and "JP-B" as used herein mean an "unexamined published Japanese patent application" and an "examined Japanese patent publication", respectively), for example, debutylation is carried out in diphenyl ether as a high-boiling solvent in the presence of p-toluenesulfonic acid at a temperature elevated to 250.degree. C.
Where a high-boiling solvent is used as described above, such a process is uneconomical and inefficient for industrial production to remove the solvent attached to the resulting crude BPL crystals. Moreover, the high-temperature reaction causes thermal deterioration of BPL, making the hue of the final product poor.
On the other hand, where a large quantity of a catalyst is used to attempt to complete debutylation under a mild temperature condition without using a high-boiling solvent, the quality of BPL, particularly the hue is deteriorated with an increase of the catalyst amount.